A communications system can include a distributed system capable of transporting signals between carriers and user devices, such as mobile devices. An example of a distributed system is a distributed antenna system that includes one or more master units in communication with carrier systems, such as base transceiver stations of cellular service providers. The distributed antenna system can include remote antenna units physically separated from the master unit, but in communication with the master unit via a serial link that may be copper, optical, or other suitable communication medium. The remote antenna units can also be in wireless communication with user devices. For example, the remote antenna units may be positioned in a building, tunnel, or other structure that prevents or limits communications directly with the carriers.
The master unit can facilitate communication between the carrier systems and the remote antenna units. For example, the master unit can down-convert and digitize via an analog-to-digital converter (ND) signals received from the carriers and can multiplex the signals into frames that are transmitted over the serial link to the remote antenna units. A signal may be one or more channels having a composite analog or digital waveform with a bandwidth that can be up to the full bandwidth of a designated telecommunication band. Examples of telecommunication bands include US Cellular, SMR800, AWS700, SMR900, EGSM900, DCS1800, PCS1900, and UMTS2100. The remote antenna units can include a digital-to-analog converter (D/A) to convert the signals to analog signals. The remote antenna units can frequency shift the analog signals to a frequency for transmission to the user devices. Communications from the user device can be similarly processed and transmitted. For example, a remote antenna unit can digitize and package the signals into frames that are transmitted via the serial link to the master unit. The master unit can convert the digital signals to analog signals at a frequency for transmission to the appropriate carrier.
The master unit can transmit several bands of signals via the serial link to the remote antenna system for distribution to various user devices. The bands of signals can be digitized using a common sample rate. The serial link, however, has a finite amount of bandwidth (e.g. 10 Gbps) available for transferring digitized signals between the master unit and the remote antenna system. Because the bands are sampled at a common sample rate (including those bands having lower bandwidth requirements), serial link bandwidth is underutilized. Accordingly, it is desirable to utilize serial link bandwidth more efficiently.
One technique to utilize serial link bandwidth more efficiently includes selecting optimal A/D and D/A sample rates for each band to be transported. For example, the master unit can include a plurality of A/Ds. Each A/D is associated with a sample rate provided by a sample clock. The sample rate for an A/D can be selected to accommodate the band of the respective signals being converted by the A/D. The corresponding D/A can use the same sample rate provided by a sample clock at the remote antenna units. The result is that the master unit transports just the required amount of bandwidth for each band at the serial bit rate of the serial link.
Implementing this technique, however, can be problematic. Generating different sample rates for A/Ds (or for D/As) can be expensive. It can also be difficult to accomplish using hardware. For example, the sample rates must be programmable, requiring low noise frequency synthesizers that can both increase costs and cause performance degradation based on phase noise introduced into the system. Furthermore, this technique requires that the front-end systems that include the A/Ds or D/As be specially manufactured or configured, limiting the ease of manufacturing and replacing these components. In addition, an anti-aliasing filter must be used that is programmable according to the bandwidth to prevent aliasing. Such programmable analog anti-aliasing filters preceding the A/D converter can be difficult to design and configure and can be expensive. Problems may also arise in interfacing with carriers due, for example, to the unavailability of an A/D using an appropriate sample rate for the bandwidth of the analog signals from a particular carrier.
Therefore, systems and methods are desirable that can utilize serial link bandwidth efficiently without requiring different sample rates for A/Ds or D/As.